42 LILLIE. [Vol. X. 



ula, in Umbrella, and in Nereis the first cleavage plane is said 

 to be transverse and the second parallel to the future median 

 plane. Why } Because it is found, when the embryonic axes 

 are determined, that the entomeres B and C are on the right 

 side and A and D on the left side. But no account has been 

 taken of the axial relations of the ectomeres. In Unio the 

 decisive factor was with me the second generation of ectomeres, 

 simply because they are the largest cells. As the four cells in 

 question lie anterior, posterior right and left, I said (No. 21) 

 that the future transverse and sagittal axes were inclined at an 

 angle of 45° to both the first and second planes of cleavage. 

 But in Nereis, Crepidula, and Umbrella the relation of the 

 second generation of ectomeres is exactly the same. In 

 Umbrella there is a fourth generation of micromeres, three 

 of the members of which are entodermal and one (the pos- 

 terior) mesoblastic. This fourth generation bears the same 

 relation to the embryonic axes that the second does. The 

 primary mesoblast M in Nereis, in Crepidula, and Unio as 

 well, lies in the middle line behind. On the other hand, in 

 Nereis, Crepidula, Umbrella, and Unio the first ^ and the third 

 generation of micromeres have the same axial relations as 

 the entomeres. Briefly expressed : the members of the odd 

 generations of ectomeres, as well as the entomeres are dis- 

 tributed, two each, right and left of the middle line ; those 

 of the even generations are placed anterior, posterior, right 

 and left. So that if the orientation be based on the odd 

 generations or on the entomeres, it will be said that the 

 first and second cleavage planes are transverse and longi- 

 tudinal respectively to the embryonic axes ; if based on the 

 second or fourth generations of micromeres, it will be said that 

 the first and second planes of cleavage are inclined at an angle 

 of 45° to both transverse and longitudinal embryonic axes. 

 Unio thus agrees completely in this respect with Nereis, Um- 

 brella, and Crepidula. 



There are forms, however, in which the relations are reversed. 



^ There is a wheel within a wheel here, inasmuch as the divisions of the first 

 generation of micromeres, being oblique, cause certain of the resulting cells to lie 

 on the longitudinal and transverse middle lines. 



